Methods used in conditioning air from one state to another state may vary widely depending on such factors as the quality of the air to be conditioned, the temperature and humidity to which the air must be conditioned, the volume of air to be conditioned, and whether a portion of the conditioned air will be outside air or recirculated air. In some applications only the temperature of the air is important whereas in other applications, the humidity of the air is more important.
The painting industry, and particularly the automotive painting industry, is one example where the humidity as well as the temperature of the air must be maintained within an optimal range of conditions. Typically in air supply units (“ASUs”) for applying a base coat, ambient air enters the unit, passes through a pre-heat burner, and then passes through both a humidification media and cooling coils. After traversing the cooling coils, the air passes a reheat burner and is blown into the booth. The use of a reheat burner provides a good method for controlling both the temperature and humidity of the air delivered to the booth. However, typical clear coat ASUs are very poor at controlling humidity. Traditional clear coat ASUs are not equipped with re-heat burners, unlike most base coat ASUs. This lack of a re-heat burner results in poor humidity control year round, especially in the late summer when humidity is high. This is problematic because high humidity in a clear coat paint booth may result in quality and delivery problems. For example, high humidity may result in high voltage kick-outs, drips from buildup, gun spit rejects, solvent pop, and frequent line stops to wipe down the booth.
In clear coat ASUs lacking a re-heat burner, a single closed control loop is generally used. The single closed control loop consists of a burner which adds heat to the airstream, a temperature sensor that reports the booth temperature, and a proportional-integral-derivative (“PID”) controller that makes adjustments to the control signal to track a fixed temperature set point. While this control loop allows for optimal temperature control in the clear coat booth, as the ambient humidity rises, the humidity in the clear coat booth may also rise to levels detrimental to proper clear coat application, thereby affecting productivity. Therefore, it is desirable to overcome the shortcomings of the traditional system and method of controlling only temperature inside a clear coat booth.